The present invention relates to the field of contactless measurements of electrical properties of semiconductors, and more particularly, to contactless measurement of the electrical properties of a semiconductor by remotely sensing a photovoltage effect induced on the semiconductor by a modulated light source by measuring resultant changes in the energy spectrum of secondary electrons which are excited by a primary electron beam that irradiates the semiconductor. In semiconductor material and semiconductor device development, it is important to be able to observe or detect the electrical properties of different regions of the semiconductor. Such properties may include surface voltage or electric field strength resulting from external stimuli. Detection of these properties is useful for assuring the quality of electrical networks implemented at various stages of fabrication processing. Electrical property variations affecting the quality of such networks are associated with dopant variations, the presence of impurities, defects, or other sample properties.
One method for detecting the electrical properties in different regions of semiconductors uses a point probe or other physical contact to make electrical measurements on the surface of the semiconductor sample material. However, there are several disadvantages with this method. The resolution of this method is limited by the contact probe dimensions which are often much greater than the regions which one may wish to probe. Another disadvantage is that this method is tedious to implement because each region to be tested must be individually contacted. Furthermore, the semiconductor surface may be damaged from contact with the probe. A serious limitation with this method is that some materials cannot be contacted by simply pressing contact probes on their surfaces. For example, gallium arsenide requires special processing to enable electrical contact with the semiconductor material. Due to the surface states of gallium arsenide, non-ohmic contact probes on the surface of the gallium arsenide result in Schottky barriers. Electrical measurements of selected regions of this material require formation of ohmic contacts by techniques which may involve evaporation and annealing of materials such as gold, germanium, and nickel on the gallium arsenide surface to form an alloy which eliminates the Schottky barriers.
In general, remote systems for measuring the electrical potential of local regions on integrated circuits are well known. These types of systems detect secondary electrons emitted from the surface of a test sample which is irradiated with an electron beam. Such techniques have been discussed extensively in the literature in articles such as "Secondary Electron Detection Systems for Quantitative Voltage Measurements", Menzel, E., and Kubalek, E., Scanning, Vol. 5, pp. 151-171, FACM Publishing, Inc. (1983).
U.S. Pat. No. 4,220,854 issued to Feuerbaum on Sept. 2, 1980 disclosed a method for contactless measurement of the potential wave form in an electronic component. A pulsed electron beam from an electron beam microscope is directed over the surface of a semiconductor sample. The pulsed radiation on the sample surface produces modulated secondary electrons. These secondary electrons are captured by an analyzer unit and guided to a detector. The signal produced by the detector is directed through a lock-in amplifier and then displayed on a picture screen.
Federal Republic of Germany Pat. No. DE 29 02 495 A1, assigned to Siemens, discloses a device for contactless potential measurement. The device uses a primary electron beam to release secondary electrons at the measurement point of an electronic component such as an integrated circuit.
Federal Republic of Germany Pat. No. DE 3331931 A1, assigned to Siemens, discloses an apparatus for measuring potential of an electronic circuit having a passivation layer. The apparatus uses electron or light scanning to perform electron beam measurement techniques. The passivation layer is scanned with low energy electrons at video frequency or with light to release electrons from the layer.
"Voltage-Contrast Detector for Scanning Electron Microscopy", by Touw, T. R., IBM Technical Disclosure Bulletin, Vol. 15, No. 8, Jan. 1973, discloses a modification to a voltage-contrast detector of the type described by J. R. Bandury and W. C. Nixon in J. Sci. Inst. Ser. 2 Vol., 2, pp. 1055-1059. The IBM device includes a planar element at specimen potential located above the specimen with an aperture to admit a primary electron beam on to the specimen and to allow secondary electrons to enter an electron detector. The aperture size may be varied to extend the voltage range of monotonic response in voltage contrast and decreases the effect on due to transverse electric fields at the specimen.
"Scanning Electron Beam Probe VLSI Chips", by Fazekas, P., Feuerbaum, H. P., and Eckhard, W., Electronics, Vol. 54, No. 14, July 14, 1981, discloses a noncontacting test method for plotting logic-state maps of the operation of integrated circuits. This method is implemented by scanning the entire surface of an integrated chip with an electron beam to utilize the well established voltage-contrast technique to produce areas of light and dark which results in an image representing logic voltage levels of the chip.
The references identified above show how it is possible to measure or observe voltages on semiconductor device circuits using the non-contacting method of electron beam irradiation and secondary electron energy analysis. However, these teachings do not address the problem of how to induce voltages into semiconductor samples for measurement. One method for exciting voltages on a semiconductor is to apply the voltages via electrical contacts physically connected to the test sample. This method is commonly practiced, but disadvantageously requires that the semiconductor sample be in a relatively finished stage of circuit fabrication. Often, it is desirous to ascertain the quality of an electrical network being manufactured on a semiconductor substrate before actual completion of the network. However, there is no present method for doing so. Therefore, a need exists for a non-contact method of determining electrical properties of semiconductor samples at various stages of fabrication so that defects of material or manufacture can be more readily identified and discovered earlier than present methods allow.